1. Field of the Invention
This invention relates to an EGR (Exhaust Gas Recirculation) control apparatus, and in particular, an EGR control apparatus for a diesel engine which can conduct a decrease control for an EGR volume in at least a high load region of the engine operating state when the engine accelerates, in order to avoid a smoke aggravation and a poor acceleration.
2. Description of the Related Art
EGR is a technology in which a part of exhaust gas is recirculated from an engine exhaust pipe to an engine combustion chamber to lower combustion temperature for reducing NOx (oxides of nitrogen) to be generated (see Japanese Patent Application published with No. 09-324673, for example).
If the EGR is performed when the diesel engine is accelerated and the engine operating state shifts into a high load region because the accelerator is depressed, since a fuel injection quantity increases under conditions that an intake air volume is relatively decreased by EGR gas rate, the intake air volume tends to be insufficient relative to the fuel injection quantity. This may cause the smoke aggravation and/or the poor acceleration. In such a case, EGR is stopped to secure a volume of fresh intake air supplied to a cylinder. The stop of EGR is performed when a target fuel injection quantity exceeds a predetermined threshold value in at least the high load region of the engine operating state (i.e., EGR stop region). The target fuel injection quantity is calculated based on an accelerator opening degree and an engine rotating speed, etc.
While the target fuel injection quantity equals to an actual fuel injection quantity to be injected from an injector, it is also used as a parameter for judging the stop of EGR. For the reason above, when the accelerator is depressed, a detected value by the accelerator opening sensor is processed by a smoothening-process in order to ease an impact at the time of acceleration, and then the target fuel injection quantity is determined based on the accelerator opening degree after smoothening-processed. When the target fuel injection quantity after smoothening-processed as such exceeds a predetermined threshold value in at least the high load region of the engine operating state (i.e., EGR stop region), EGR is stopped, and simultaneously, a corresponding quantity of fuel to the target fuel injection quantity is injected from the injector.
Here, in a fuel injection system in recent years, as is obvious by the fact that the injector can adapt to a pilot injection or a multi injection, the quantity of the fuel injected from the injector can be rapidly increased in response to an increase of the target fuel injection quantity after smoothening-processed as described above. However, with regard to the stop of EGR, since it is carried out by dosing an EGR valve provided in an EGR passage which connects the engine exhaust pipe with the engine intake pipe, a certain amount of time lag is unavoidable which is caused by a mechanical time lag and an airflow lag.
It is now assumed that actuators for the injector and the EGR valve are respectively Inputted with the increase signals of the target injection quantity after smothening-processed, and the increase control for the fuel injection quantity and the dosing control for the EGR valve are conducted based on these signals. The injector is quick to respond to the increase of the target fuel injection quantity after smoothening-processed, and can promptly inject the actual, increased quantity of fuel corresponding to the target fuel injection quantity. However, this increase of the fuel injection quantity precedes the stop of EGR, because a mechanical actuation lag, the airflow lag, and the lag caused by the smoothening-process simultaneously occur in the EGR valve actuator side. As a result, in practice, EGR is not stopped for a certain period of time since the actual fuel injection quantity is increased. For this reason, during the certain period of time (EGR stop lag time), the intake air volume tends to be insufficient relative to the fuel injection quantity, and the smoke aggravation and/or the poor acceleration may occur.